Optical discs are of great interest as high capacity information carrying media. Among the optical discs, magneto-optical discs of the magnetic field modulation system are expected to find use in data files or the like. A magneto-optical disc is recorded in a magnetic field modulation mode by continuously irradiating a laser beam from an optical head to the recording layer of the disc for raising the temperature thereof and simultaneously applying a modulated magnetic field across the recording layer from a magnetic head opposed to the optical head. Therefore, the magnetic field modulation mode allows for overwrite recording.
Most prior art magneto-optical disc drives rely on the contact-start-and,stop (CSS) system using a flying magnetic head. The magnetic head is in contact with the disc surface at the start and end of disc rotation. Then the disc on the surface facing the magnetic head is provided with a protective coat for prohibiting adsorption of the magnetic head and crashing.
Recently a magneto-optical disc which allows for writing and reading at the same linear velocity as the compact disc (CD) was developed and put into commercial practice as a mini-disc (MD). The CSS system cannot be applied to the MD because the MD is operated at a low linear velocity of 1.2 to 1.4 m/s so that the magnetic head cannot be kept afloat. Also since the magnetic head can produce only a very weak magnetic field for various limiting factors, the magnetic head must be placed as close to the recording magnetic layer as possible. For these reasons, the magnetic head used with the MD is of the type which is kept in contact with the magneto-optical disc during writing, that is, the sliding contact type.
Writing by a sliding contact type magnetic head suffers from problems of scratching and wear of the recording magnetic layer by sliding contact because the magneto-optical disc are in continuous contact with the magnetic head. It is then necessary to provide a protective film on the magnetic head facing surface of the magneto-optical disc for protecting the recording magnetic layer. Such protective films are conventionally formed of radiation-cured resins, especially UV-cured resins. The radiation-curable resin composition and cured film thereof have many advantages including ease of application to polycarbonate substrates, firm adhesion, low moisture permeability, and higher hardness than polycarbonate.
There still remains the risk that the protective film can be scratched during sliding contact with the magnetic head, eventually leading to failure of the recording magnetic layer.
It was then proposed to provide a magneto-optical disc on its magnetic head facing surface with a wear resistant protective film having a roughened surface or a wear resistant protective film of UV-cured filler-loaded resin. Known wear resistant protective films include a sheet-like sliding protective film having a pencil hardness of at least 2H and a surface roughness Ra of 2,000 to 4,000 .ANG. (Japanese Patent Application Kokai (JP-A) No. 28559/1993), a protective layer loaded with particles having a Vickers hardness of 1,300 to 3,000 kg/mm.sup.2 (JP-A 36131/1993), a protective layer whose surface is roughened to a surface roughness Ra of 0.01 to 0.04 .mu.m by burnishing (JP-A 303784/1994), and a protective film including a protective layer of a UV-cured resin and a wear resistant protective layer of a UV-cured resin loaded with silicon dioxide particles (JP-A 210877/1993). It was also proposed to roughen a surface of a wear resistant protective film by blasting abrasives (JP-A 195943/1992) or by burnishing (JP-A 195749/1992), to control the surface roughness Rmax within a specific range (JP-A 195748/1992), and to form a laminate including an undercoat layer with a predetermined Rmax and wear resistant protective layer (JP-A 195746/1992). These wear resistant protective films are still unsatisfactory in resistance to abrasion and scratch and thus less durable.
It was also proposed to form a wear resistant protective film by adding lubricant to a UV-curable resin, followed by coating and UV curing. Alternatively, a protective film of a UV-cured resin is coated on the surface with a lubricant. For example, in JP-A 303784/1994, a protective layer is roughened to a surface roughness Ra of 0.003 to 0.05 .mu.m by burnishing and then top coated with a lubricant. JP-A 242542/1993 discloses an overcoat layer comprising a UV-cured resin, diglycerin tetralaurate and optionally, isodecyl adipate. JP-A 210883/1993 discloses a protective film formed from a composition comprising a photo-curable resin based on urethane acrylate and a suitable amount of a PO/EO modified silicone of a specific structure. However, these protective films can be scratched by sliding contact with a magnetic head, particularly in a dusty environment, unless the UV-cured resin has satisfactory properties.
Other means for enhancing the durability of magneto-optical discs include cleaning of a wear resistant protective film on its surface as disclosed in JP-A 195944/1992. It is also known from JP-A 195747/1992 to treat an underlying surface with an electric discharge before forming a Wear resistant protective film thereon for the purpose of enhancing the adhesion of the protective film.
However, all these protective films are not satisfactorily durable against sliding contact with the magnetic head. There still remains the problem that the protective film can be damaged.
For a recently developed MD data system utilizing MID as an external memory of a computer, there is a need for a more durable protective film.